An atomic force microscope (AFM) is a measurement instrument that can be used to map-out various attributes of a small (e.g., nanoscopic) sample, such as its surface topography or material properties. For example, an AFM can be used to capture a nanoscale image of an integrated circuit or to measure nanoscale stiffness variations of a blended polymer surface.
A typical AFM comprises a mechanical probe tip connected to a cantilever arm. The cantilever arm is controlled by one or more actuators to raise and lower the probe tip with respect to the sample and/or to vibrate the cantilever arm with a desired pattern. During typical operation, the probe tip is moved across a sample surface, and mechanical interactions between the probe tip and the sample produce deflection of the cantilever arm. This deflection is then detected and used to quantify one or more attributes of interest. For instance, the deflection can be transformed into a quantitative measurement of some physical attribute of the sample through the use of one or more nanomechanical models.
One way for an AFM to measure the material properties of a sample is through the use of force-height curves. A force-height curve is a measurement that indicates the magnitude of force between the probe tip and the sample as a function of some height measurement, such as the height of the actuator. For instance, a force-height curve may be generated for a single location of a sample by detecting cantilever deflection while lowering the actuator until the probe tip touches down at that location, continuing to lower the actuator so that the probe tip presses against the sample with increasing force, and then raising the actuator until the probe loses contact with the sample. To map-out the material properties of a sample, this process may be repeated multiple times for each of multiple locations of the sample.
As the probe tip presses against the sample, the cantilever may deflect (e.g., bend) according to the stiffness or other material properties of the sample. For instance, where the sample is relatively soft, the cantilever may bend by a relatively small amount, and where the sample is relatively stiff, the cantilever may bend by a relatively large amount. The cantilever deflection during this part of the measurement can be used to determine the sample stiffness or some other material property at that location.
Subsequently, when the actuator is raised and the probe tip loses contact with the sample, the cantilever arm may begin to vibrate, or ring, due to an adhesive force between the probe tip and the sample. For instance, the sample may have a certain amount of stickiness or water tension that may adhere to the probe tip, so in order to lift the probe tip from the sample, the cantilever arm must build up enough energy to overcome the stickiness or water tension. Once enough energy is built up, the probe tip will “snap-off” the sample and vibrate back and forth until the vibration decays through a natural damping process. This damping process may last a relatively long time when the cantilever arm is operating in air because air has a relatively low damping coefficient.
For most applications, the vibration or ringing of the cantilever arm is not considered to provide useful information. For example, the ringing tends to obscure any measurable probe tip-sample interactions, so it is generally not useful for determining material properties of the sample. Nevertheless, the vibration cannot simply be ignored because it can potentially damage the sample. Accordingly, it is usually necessary to wait for the vibration to decay before performing a next force-height measurement.
Unfortunately, this waiting process can significantly slow the process of mapping-out material properties using force-height curves. Accordingly, there is a general need for techniques to address the vibrations or ringing in order to improve the speed of generating force-height curves.